Latex-based paints have captured a significant portion of the indoor and outdoor paint market as a result of the many advantages that such paints have over solvent-based products. The main advantage of latex-based paints include easy clean up, low odor and fast dry times.
The properties that are desirable in paints, namely the ability to be used at a temperature low enough for application over a long seasonal range, to withstand repeated cycles of freezing and thawing, and to form a film hard enough to avoid tackiness, blocking and dirt pickup in the intended application, are enhanced in latex based formulations by the addition of coalescing solvents and anti-freeze agents. These coalescing solvents (for example, butyl carbitol acetate) and anti-freeze agents (for example propylene glycol, ethylene glycol) are volatile organic compounds that are present in amounts up to 360g per liter of paint (3 lbs. per gallon), not including water.
Coalescing solvents are needed because the polymers used in latex paints must have the lowest possible minimum film forming temperature (MFFT) and the highest possible glass transition temperature (Tg). The MFFT is the lowest temperature at which the polymer particles will mutually coalesce and form a continuous film when the water, which is the solvent base, evaporates. Polymers that have low MFFT extend the temperature conditions under which the paint can be applied. The Tg is the temperature at which a polymer changes from an amorphous, soft and tacky state to a glossy, hard and rigid state. Polymers with high Tg values will result in a paint coating that will be hard, resistant to abrasion and resistant to blocking. Coalescing solvents effectively lower the Tg of the polymer to meet the desired low MFFT on application, and then eventually diffuse out of the paint and evaporate, leaving a high Tg film. Antifreeze agents are added to paint formulations simply to impart freeze-thaw stability.
In U.S. Pat. No. 4,906,684, ambient curing coatings which contain an acetoacetoxy group (obtained via copolymerizing unsaturated acetoacetoxy groups), a glycidyl group and a carboxylic acid group are disclosed. Also disclosed are film forming acrylic polymers which contain pendant acetoacetoxy groups, glycidyl groups and carboxylic acid groups.
In U.S. Pat. No. 4,408,018, polymers containing pendant acetoacetate moleties are mixed with polyacrylates which contain more than one unsaturated acrylic group and cured through Michael addition using a strong base as catalyst.
Coating compositions which cure under ambient conditions have been known for a long time. The earliest of such coating compositions are coatings based on drying oils (alkyds) which cure under auto oxidation. Other coating compositions which have been developed more recently, are those based on the epoxide-carboxylic acid reaction, isocyanate-moisture reactions, polyaziridine-carboxylic acid reaction, and activated methylene reactions.
With the universal recognition that volatile organic compounds are detrimental to the environment, there is a need for latex-based paints that contain no coalescing solvents or anti-freeze agents. Considerable research continues in an effort to create solvent-free coating systems with superior abrasion properties. Soft binders designed for solvent-free systems exhibit poor abrasion properties when compared to hard binders intended for use with fugitive coalescents. There is an urgent need to design latices for VOC free coatings exhibiting abrasion properties comparable to solvented, water-based systems. The present invention thus provides certain compounds which have been found to serve as reactive materials in latex compositions for enhancing abrasion resistance in formulated pigmented coating systems without adding to the volatile organic compound content.